Prodder with force feedback

ABSTRACT

A detector equipped with force feedback for detecting detonatable devices such as land mines is disclosed. The detector includes a rod having a tip for placing beneath the surface of the ground and for contacting unknown objects. The tip ultrasonically provides an indication of the presence of a metal or plastic material. Another rod within the device is used to compensate the device during use against unknown pressures applied to the rod beneath the surface of the ground.

FIELD OF THE INVENTION

[0001] This invention relates generally to prodders for probing theground for buried explosive devices such as landmines and the like, andmore particularly to a method and device for providing force feedback tothe prodder and/or the user of the device.

BACKGROUND OF THE INVENTION

[0002] Despite a variety of mechanised means now available for detectingand clearing landmines, the current hand tool of choice is the handprodder. Personnel exhibit greater confidence when traversing aminefield which has been hand-prodded by their compatriots than they dowith fields cleared by other means.

[0003] The traditional hand prodder typically comprises a 30 cm longpointed rod extending from a gripping handle. The probe is generallynon-magnetic to avoid setting off magnetically-triggered mines. The userprobes the ground ahead and excavates any hard objects which the probecontacts. As the ratio of rocks to landmines in a minefield may number1000:1, excavation of every contact is labourious, but very necessary.

[0004] Currently, instrumented prodders are known having ultrasonicmeans in the form of an ultrasonic transducer at or near the probe tipthat are used for characterisation of buried obstructions. These devicescan be used in conjunction with a minimum metal content (MMC) detector,wherein the MMC detector first detects the ground indicating thevicinity of a land mine, and, wherein the instrumented prodder is usedto probe the earth in the vicinity of the suspected land mine, thelocation of which may have been isolated using the MMC detector. MMCmine detectors having a search head and circuitry for detecting buriednonmetallic and metallic land mines are well known. For example, U.S.Pat. No. 4,016,486 in the name of Pecori assigned to the United Statesof America by the Secretary of the Army, hereby incorporated byreference, discloses such circuitry. U.S. Pat. No. 5,754,495 toGallagher, hereby incorporated by reference, discloses an instrumentedprodder having a probe in the form of an elongate, preferablynon-magnetic rod including a gripping handle disposed at one end. Thedesign of the probe is based partially upon a Split Hopkinson PressureBar (SHPB) apparatus. In the apparatus, a compression wave or highfrequency elastic mechanical pulse is delivered via a rod to a sample,wherein a portion of the wave is reflected. The incident wave launchedat the sample is reflected and/or transmitted from or through thesample, respectively, in dependence upon the characteristics of thematerial. The effect of mechanical impedance, which is a characteristicof a material, on a SHPB apparatus in three instances is describedhereafter:

[0005] Firstly and obviously, if the mechanical impedance of a sampleunder test is the same as that of an incident bar in the SHPB, therewill be no reflection as the sample will be displaced in a same manneras the bar itself as the compression wave is delivered. The displacementof the end of the bar is directly proportional to the strain measured(ε).

[0006] Secondly when the mechanical impedance of a sample isconsiderably greater than that of the bar, a sample's mechanicalimpedance tends toward being infinite and substantially the entire waveis reflected.

[0007] In a third instance when the mechanical impedance is zero, in theabsence of a sample, the reflected wave is tensile but of equalmagnitude to the incident wave. The phase of the wave is shifted by πand the net stress is zero; the relative displacement at the bar endequals twice that for the first instance (2ε).

[0008] In a SHPB device, once the relative displacement of the bars isknown, the displacement of the sample is ascertained. Taking intoaccount Young's Modulus (E) and the displacement of the bar, the imposedstress can be calculated, wherein the force applied is equal to theproduct of the stress and the cross-sectional area of the bar.

[0009] Since the loading on the sample becomes equal after a short time,the analysis may be somewhat simplified. Strain results may be used foronly the incident bar; or alternatively, the striker bar may be directedto impact directly on the sample, and the transmitter bar alone may beused to define the sample characteristics.

[0010] It is has been found that plastics, minerals and metals may bediscerned from one another by using this approach.

[0011] It has been further found that the hand held prodder disclosed byGallagher having a rod modified to be analogous to the incident bar of aSHPB may be used to detect or discern metal, plastic and rocks.

[0012] The prodder rod is provided with one or more piezoelectrictransducers capable of generating an acoustic wave into the rod and fordetecting reflected waves from an object contacting the end of the rod.Conveniently, signal processing means are coupled to the transducers andare provided for analysing the detected reflected waves for determiningthe characteristics of the object; more especially, for distinguishinglandmines from inert rocks. The signal processor establishesmeasurements of the frequency-time-amplitude characteristic of theobject. The reflected waves are compared with known characteristicsignatures of a plurality of materials to attempt to ascertain a matchwithin predetermined limits.

[0013] Although U.S. Pat. No. 5,754,495 describes a device that performssatisfactorily in many instances, it suffers from a problem related tothe fact that acoustic coupling at the obstruction is a function of theforce applied to the probe end. As a result, the results are oftenerroneous. This is particularly detrimental when the prodder indicatesthat the obstruction is a rock, when in fact it is a land mine.

[0014] Preferably, enough force will be applied to the probe end suchthat characterisation of the obstruction can occur without causingdetonation; and, preferably, a relatively consistent force will beapplied to the probe end such that an accurate determination as to thecharacter of the buried obstruction can be made. However if too littleforce is applied at the probe end, a poor reading may result and a minein the vicinity of the probe may go undetected. Too much force appliedat the probe end in the vicinity of a land mine may inadvertentlydetonate the mine.

[0015] Mechanical force sensors such as springs suffer from severaldisadvantages. Firstly, they are subject to fatigue over time. Secondly,they are often difficult to design such that they are robust enough andaccurate enough for military applications without incurring significantcosts. These drawbacks are well known and overcoming them would beadvantageous.

[0016] It is therefore an object of the invention to provide a methodand device, which will overcome the aforementioned problems, related totoo much force, too little force, or a varying force being applied tothe probe end while in use.

[0017] It is a further object of the invention to provide aninstrumented prodder for detection of land mines and the like thatincludes force feedback for sensing a force, such as pressure, appliedto an end thereof.

[0018] It is another object of the invention to provide an instrumentedprodder for detection of land mines and the like, that provides datarelated to characteristics of the probed object that are independentfrom the force of the prodder on the object.

[0019] It is a further object of the invention to provide a hand-heldprodder for probing the ground for buried explosive devices such aslandmines and the like, that is relatively simple, rugged, andinexpensive.

SUMMARY OF THE INVENTION

[0020] In accordance with the invention there is provided a prodderhaving force feedback for detecting detonatable devices or land mines,comprising:

[0021] a rod having an end for placing in contact with a first object tobe detected;

[0022] a first transducer coupled to the rod for providing an acousticwave to the first object and for receiving acoustic waves reflected fromabout the first object;

[0023] a second rod having an end for placing in contact with areference object;

[0024] a second transducer coupled to the second rod for providing anacoustic wave to the reference object and for receiving acoustic wavesreflected from about the second object; and,

[0025] an electronics module for analysing data related to the wavesreflected from the reference object to determine a value related to theforce applied to the first rod, for analysing data related to the wavesreflected from the first object in dependence upon the determined valueto determine acoustic characteristics of the first object material so asto categorize the first object's material in a force independentfashion.

[0026] In accordance with another embodiment of the invention there isprovided a land mine detector having force feedback comprising:

[0027] a rod having a sensing end for contacting objects buried beneaththe ground;

[0028] a transducer coupled to a non-sensing end of the rod forimparting mechanical energy into the rod towards the sensing end incontact with a buried object, and for detecting a signal correspondingto reflected waves from the buried object, the signal containinginformation related to a force exerted on the rod and to materialcharacteristics of the buried object;

[0029] a second transducer coupled to an end of a second rod forimparting mechanical energy into the second rod towards another end incontact with an object having known characteristics, and for detecting asignal corresponding to reflected waves from the second object, thesignal containing information related to the force exerted on the rodand to material characteristics of the known object; and,

[0030] means for processing the signals, for determining at least one ofthe force exerted on the rod, and force-independent materialcharacteristics of the buried object for comparing with stored materialcharacteristics of known objects so as to categorize the object'smaterial.

[0031] In accordance with yet another embodiment of the invention thereis provided a prodder having force feedback for detecting detonatabledevices or land mines, comprising:

[0032] a rod having an end for placing in contact with a first object tobe detected;

[0033] a first transducer coupled to the rod for providing a firstacoustic wave to the first object and for receiving first acoustic wavesreflected from about the first object and for providing a secondacoustic wave to the first object and for receiving second acousticwaves reflected from about the first object; and,

[0034] an electronics module for analysing data related to the first andsecond acoustic waves reflected from the first object to determineacoustic characteristics of the first object material so as tocategorize the first object's material in a force independent fashion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Exemplary embodiments of the invention will now be described inconjunction with the drawings, in which:

[0036]FIG. 1 is a prior art circuit diagram of a Split HopkinsonPressure Bar device;

[0037]FIG. 2 is a prior art cross-sectional view of a hand proddercontacting a sub-surface object;

[0038]FIG. 3 is a prior art cross-sectional view of the rod andpiezoelectric crystal portion of the prodder coupled to the signalprocessing module;

[0039]FIG. 4 is a prior art flow chart of the digital signal processorand A/D functions;

[0040]FIG. 5 is a cross-sectional view of the rod and piezoelectriccrystal portion of the prodder with force feedback;

[0041]FIG. 6 is an exploded perspective view of the prodder componentsdisposed within a sleeve;

[0042]FIG. 7 is a cross-sectional view of the hand prodder contacting asub-surface object, according to an embodiment of the invention; and,

[0043]FIG. 8 is a cross-sectional view of a multi-frequency ceramicacoustic transducer.

DETAILED DESCRIPTION

[0044] In prior art FIG. 1 a specimen sample is shown juxtaposed betweenan incident bar and a transmitter bar. A strain gauge disposed on eachbar provides a signal to signal processor as is described heretofore.

[0045] In prior art FIG. 2 a hand-held prodder for probing the groundfor buried explosive devices such as landmines and the like is provided.The prodder comprises a rod 2 having a first end 3 flexibly supported byan annular rubber coupling 4 in a mounting nub 5. The nub 5 is screwedinto a handle 6. The rod has a pointed second end 7 for sensing objects8 buried in the ground 9.

[0046] The rod 2 is 45 cm long and is formed of non-magnetic, austeniticstainless steel. Only 30 cm project from the rubber coupling 4. Therubber coupling 4 lessens the rigidity between the rod 2 and handle 6.

[0047] Best seen in prior art FIG. 3, a piezoelectric crystal 10 isglued to the first, or driver end 3 of the rod 2. When an electric fieldis applied to the crystal 10, a mechanical strain will occur and drivemechanical energy into the rod's driver end 3. Conversely, when thecrystal 10 is mechanically stressed, an electric charge is produced. Asuitable crystal is a 15 mm long, 6.35 mm diameter poly-crystallineceramic cylinder, model Sonex P-41 available from Hoechst CeramTec,Mansfield, Mass. The crystal 10 is electrically insulated from the rod 2with a ceramic insulator 11. Optionally, the insulator further serves toprovide mechanical strength to the joint between the crystal and therod.

[0048] Positive and negative electrical leads 12 from the crystal passthrough the nub 5 for bidirectional electrical signal transmissionbetween the crystal 10 and an electronics module 13. Shown in FIG. 2,the module 13 is installed within the prodder's handle and is poweredwith 9 V batteries 14.

[0049] The electronics module 13 is capable of two modes: a driver modeand a signal processing mode. In the driver mode, an electrical signalis transmitted along leads 12 to the crystal 10 for generating apiezoelectric mechanical pulse. The pulse is introduced into the rod'sdriver end 3. In the signal processing mode, any electrical signalsgenerated by the crystal 10 are transmitted along leads 3 for processingby the electronics module 13.

[0050] More specifically, the module 13 comprises a digital signalprocessing microcomputer 15, an EPROM 16 containing program instructionsand digital storage means, an A/D converter 17, a signal input amplifier18 and a driver output amplifier 19. An audio/visual binary outputdevice 20 is provided.

[0051] A suitable signal processor is a model ADSP-2181 digital signalprocessing microcomputer by Analog Devices, Inc., Norwood, Mass. TheADSP-2181 contains a high speed serial port, 16 bit data processingcapabilities and has both onboard program RAM and data memory RAM. Forpermitting battery powered operation, the ADSP-2181 features a powersaving “sleep ” mode. After downloading of program instructions from theEPROM, the ADSP-2181 will reduce its power consumption and await asuitable trigger before “waking-up ” to begin signal processing. Havingreference to the prior art flow chart in FIG. 4, when the prodder isactivated, the EPROM 16 downloads the analysis program to the ADSP-2181processor 15 and awaits a trigger. When triggered (i.e., contact of therod's sensing end with an object) the EPROM 16 signals the driver outputamplifier 19 to generate an ultrasonic analog driver signal (20-200kHz). The driver signal stimulates the crystal 10 to generate amechanical pulse and send it as an acoustic incident wave down thelongitudinal axis of the rod 2. The incident wave reflects from theobject 8 at the rod's sensing end 7 and returns to the rod's driver end3 as a reflected wave. The mechanical energy in the reflected wavestimulates the crystal 10 to generate electrical analog signalscharacteristic of the reflected wave.

[0052] The analog signals are processed through the signal inputamplifier 18 and converted by the A/D converter 17 for analysis by thesignal processor 15. A suitable A/D converter is available as modelAD876 10 bit, 20 MSPS (million samples per second) CMOS converter, alsofrom Analog Devices, Inc. The AD876 is also capable of a “sleep ” mode.

[0053] The digital processor 15 stores the reflected data in its RAMmemory. The characteristics of the reflected signal are dependent uponthe material characteristics of the object 8. Different materials havedifferent MI and frequency-dependent damping coefficients. Analysis ofthe reflections and damping rates demonstrated in the reflected data isinstructive of the material characteristics of the object.

[0054] Accordingly, using one analytical technique, the stored data isconditioned using a stepping FFT and analysed forfrequency-time-amplitude information. A 256 point FFT from a 1024 sampleis advanced in 128 sample steps which yields 7 time-slices of FFtransformed data. The characteristics distinctive of the material aregenerally located within the first 5-10 harmonics or bins of thetransformed data.

[0055] The effects of the peculiar characteristics of the rod arecalibrated by causing the piezoelectric crystal to send a pulse alongthe rod when its sensing end is not contacting anything. This “dry-fire”provides a baseline reading which accounts for individualcharacteristics including the tapered point of the bar, wear,temperature, and accumulated debris. This resulting baseline power datais subtracted from the actual contact data.

[0056] Non-contact calibration can be done before each use to accountfor physical prodder variations. The extraction of the baseline rodcharacteristics heightens the sensitivity of the signal analysis, havingremoved a portion of the signal which is not attributable to the object.

[0057] However, non-contact calibration does not account for variationsin pressure with which the sensing end 7 of the rod is forced againstthe object 8 to be detected. In fact, there is no attempt to calibratethe prodder with respect to effects from an applied force, such aspressure. This is a significant limitation to the prodder describedheretofore and shown in the figures. Since readings acquired with theprodder are dependent upon applied pressure, and since the appliedpressure is likely different each time the prodder is used, it isdesirable to provide means for providing force feedback to compensatethe readings for an applied force. Providing force feedback to theprodder and/or the user of the prodder also allows the applied force tobe determined to calculate whether too little or too much force isapplied to the object being detected. Preferably, the means forproviding force feedback does not reduce the durability of the prodderand/or significantly increase the manufacturing cost.

[0058] Referring now to FIG. 5, an embodiment of a prodder having forcefeedback is shown. The device and method for providing force feedbackare comparable to the device and method described heretofore. Theprodder includes a probing rod 2, a housing 12 for receiving a non-Docprobing end of the rod 2, and a threaded lock fitting screw 5 having abore extending through its shaft for slidably receiving the rod 2 andfor securing the rod 2 within the housing 12. A spacer in the form of acompressible washer 22 is disposed between an inside face of a flange ofthe threaded probe mount 5 and an outside face of the housing 12.Compression fitting 4 electrically insulates the non-prodding end of therod, which is coupled to a first transducer 10.

[0059] The ‘force sensing’ section of the prodder 1 includes a secondtransducer 30 linearly coupled to the first transducer 10. The first 10and second 30 transducers are separated by an acoustic insulator 32. Asecond rod 34 couples the second transducer 30 to a known object 36.First transducer 10, second transducer 30, and known object 36 areconfigured so that when the prodding end 7 is forced against anobstruction, each experiences an equivalent force applied thereto. Theprodder also includes an electronics module 13 disposed in a prodderhandle 6 for controlling the ultrasonic transducers, for analysing theacquired signals, and for determining material characteristics of theobject independent of the applied force to the object.

[0060] In operation, the prodder 1 in accordance with the invention,works in the following manner: The prodder rod 2 is inserted into theground 9 until it comes into contact with an object 8. The force exertedby the user to push the rod 2 into contact with the object is met withan approximately equal but opposite force of the rock 8 on the rod'ssensing end 7, provided the rock 8 does not substantially move. Thisforce is relayed to the components of the prodder that are linearlycoupled to the rod 2 and the object 8. For example, if the force appliedto the rod's sensing end 7 is directed towards the driving end 3 of therod, then it will be relayed through the intermediate and linearlyarranged components in a manner such that the known object experiencesan equivalent force applied thereto. Alternatively, depending upon theconstruction of the probe, the force experienced by the known object isproportional to the applied force.

[0061] The electronics module induces the first 10 and second 30transducers to launch approximately simultaneously ultrasonic pulsestowards the prodding end 7 and the non-Doc prodding end 38,respectively. An ultrasonic pulse from the first transducer 10 travelsthrough the rod 2 to the unknown object 8 in contact with the sensingend of the rod 7 and is reflected back to the first transducer 10, whereit is converted to electrical signals indicative of rod 2 and theenvironment about rod 2. For example, the electrical signals aretypically related to both the material characteristics of the unknownobject being detected and the force applied to the probing end of therod 7.

[0062] The ultrasonic pulse generated by the second transducer 30travels through a second rod 34 to a known object 36 in contact with thesecond rod 34, and is reflected back to the second transducer 10, whereit is converted to electrical signals indicative of rod 34 and theenvironment about rod 34. These electrical signals are typically relatedto both the material characteristics of the known object 36 and theforce with which the prodder is forced against the object 8. Since theenvironment and material characteristics about the known object 36 areknown and relatively constant, variations in reflected data returning totransducer 30 are mostly dependent on variations with the applied force.

[0063] The electronics module 13 processes, stores, and analyses thereflected data from the first 10 and second 30 transducers, as describedabove. In particular, the reflected data received at the secondtransducer 30 is used to compensate the reflected data received at thefirst transducer 10. For example, the reflected wave returning to thesecond transducer 30 is converted into a corresponding electrical signalrelated to the applied pressure and is subtracted from a signalcorresponding to the reflected wave returning to the first transducer 10related to the applied pressure and the material characteristics of theunknown object 8, so as to produce a signal representing substantiallythe material characteristics of the unknown object. In effect, thedesired results are extracted or deconvoluted from thepressure-dependent readings. The resulting compensated data provides apressure-independent reading, i.e., the reading that would be acquiredif there was no force applied to the prodding end of the rod 7. Thecompensated data is used to categorise broadly the unknown object asplastic, rock or metal. The user is presented with a visual indication,preferably in the form of a light pattern indicating the type ofobstruction.

[0064] This method of determining the characteristics of and classifyingthe unknown object is more accurate than methods not accounting forvariations in applied force. Since the transducer 10 used in determiningthe characteristics of the unknown object and the transducer 30 used asa ‘force sensor’ are similar or identical, they also have or experiencesimilar temperature dependencies, wear due to material fatigue,durability, and/or variations in external environment. Moreover, sincethe force sensor is constructed generally from the same materials usedin constructing the prior art device, the prodder with force feedback isconstructed with minimal additional costs.

[0065] The force applied to the rod 2 is easily calculated to provide anindication of the applied force to the user and/or the prodder. Inaddition to providing means for compensating the reflected data forvariations in applied force, the magnitude of the applied force providesthe user with information regarding the pressure they are applying tothe unknown object. The latter is of particular importance when the userneeds to apply a force that is high enough to provide a reliablereading, but not high enough to accidentally detonate a landmine. In oneembodiment, a signal indicative of the applied force is used to sound analarm when too much or too little force is applied. Alternatively, theintensity of the alarm increases and/or decreases, dependent upon theamount of force applied. In another embodiment, a visual indication ofthe applied force is provided, i.e., in the form of a plurality of LEDsor similar indicators.

[0066] As described in the prior art, the rod is preferably formed froma non-magnetic, austenitic stainless steel and the transducer is anappropriate piezoelectric crystal. The known object 36 is constructedfrom a material, such as an appropriate plastic or metal, with uniquematerial characteristics. Rod 34 is constructed from the same materialused to construct rod 2, or some other acoustically conductive material.Optionally, the rod 34 is tapered at the end contacting the known object36. Alternatively, transducer 30 is directly coupled to known object 36in the absence of rod 34.

[0067] In another embodiment, shown in FIG. 6, the rod 2 coupled to thefirst transducer 10 is the same shape and size as the rod 34 coupled tothe second transducer 30. This has the advantage that the reflectedsignals correspond to data acquired at approximately the same time, andis perhaps more accurate.

[0068] The acoustic insulator 32 is constructed preferably of amaterial, such as an appropriate rubber, that shields the secondtransducer 30 from the acoustic waves originating from transducer 10,but allows the force applied to the sensing end 7 of the rod to berelayed to the force sensing components. In some instances a degree oftorque will also be compensated for.

[0069] The acoustic pulses generated in the first 10 and second 30transducers generally have the same pulse duration and frequency.However, in some circumstances it is advantageous for the pulse durationand/or pulse frequencies to differ. For example, if the acousticinsulator does not effectively block the acoustic waves in the proddingsection of the probe from the components in the force-sensing section ofthe probe, these differences can be used to filter out the desiredmaterial characteristics of the unknown object.

[0070] In order for the acoustic waves to be transmitted withoutdistortion, the coupling between components, such as the transducer andthe rod, must be free of imperfections such as interruptions (airpockets) or resonance impeding contacts (such as screws or welds) thatdampen the transmission. There are various means, such as an appropriateadhesive, of securing each of the rods to the corresponding transducer.In one embodiment, shown in FIG. 7, the acoustic insulator 32 serves asa barrier between transducers within a pliant, yet stable, sleeve 40.The sleeve also maintains longitudinal alignment between the components,i.e., the rods, the transducers, and the known object 36.

[0071] As described in the prior art, the effects of the peculiarcharacteristics of the prodder are easily accounted for by producing a“dryfire”, which provides a baseline reading that accounts forindividual characteristics including the shape, length, temperature, andcondition of the rods 2 and 34, the transducers 10 and 30, and anyintermediate components or adhesives. The resulting baseline data issubtracted from the pressure-compensated contact data.

[0072] There are many advantages of the device as described heretofore,as compared to other devices with force feedback. One of the mostsignificant advantages is the low cost of the device. Since thetransducers serve as both the probing means and the force sensing means,no extra parts are needed. The limited number of parts makes the devicevery simple and economical to manufacture. The fact that the forcesensing components are disposed within the prodder, makes the proddermore rugged, durable, and compact.

[0073] Additional cost considerations are recognized since the probingand force sensing means wear along the same time scale, and thus do notneed to be replaced at different times. Furthermore, since additionalcomponents are not necessary, the range of the prodder is not limited tospecialized parts. For example, the thermal sensitivity is limitedprimarily by the transducers.

[0074] Other advantages relate to the unique arrangement of thetransducers. Since the first and second transducers are linearlyarranged, the force applied to the tip 7 of the probe is equallyconveyed to the first 10 and second 30 transducers, and to the knownobject 36. Accordingly, the effect of the applied force is easilyfactored out from the acoustic waves reflected from the unknown object8. Since the first 10 and second 30 transducers are simultaneouslyenergized, the wave reflected from the known object 36 can be used as abaseline reading to provide a more accurate characterisation of theunknown object 8. For example, in addition to variations in appliedforce, variations in temperature, wear, and battery power are accountedand automatically compensated for.

[0075] In its broadest embodiment, the invention relies on at least twodata sets to provide independent correlations between applied force andmaterial characteristics. The resulting correlations can then be used todetermine a solution. As described above, when applied force and a firstobject material characteristics are unknown and a second object materialcharacteristics are known, the applied force is solved based on thesecond object and used to compensate for the material characteristicdetermination for the first object. This is straightforward.

[0076] Though it is less easily envisioned, when the resulting systemhas two variables and two independent equations, the system is alsosolvable. Thus if applied force is a same value in each data set andobject material is known to be identical, then the only issue remainingis equation independence. As long as two methods of determining theforce and material characteristics are used that are independent, theresulting equations are solvable.

[0077] In an alternative embodiment, two probing rods are used eachcoupled to a transducer. Optionally, four data sets are captured—onewith the first rods transmitted accoustic signal and another with thesecond rods transmitted acoustic signal. Alternatively, the second rodonly has a receiver for receiving an acoustic signal transmitted alongthe first rod. Further alternatively, each transducer is operatedindependently to provide two data sets.

[0078] It is highly advantageous that a same electronics module is usedfor processing signals associated with each received acoustic signal.Typically, the module consists of a single processor for analysing eachreceived acoustic signal. Thus, additional costs are not incurred inimplementing the processing for both rods.

[0079] It is also possible to probe with multiple frequencies using asame rod. This is particularly useful when material signatures vary overa frequency range. For example, thin metal housings are difficult toidentify. High frequency probing is useful for distinguishing thin metalsamples which are difficult to identify with lower frequencies.Unfortunately, these same high frequencies are not the most suitablefrequencies for operation with the present invention. As such, the useof a first near optimal frequency followed by a higher frequency signalallows for identification of thin samples more accurately.

[0080] In order to probe with two or more frequencies using a singlerod, a multi-frequency transducer is described with reference to FIG. 8.A ceramic acoustic transducer in the form of a piezoelectric transduceris shown. The transducer is formed of several sections shown here asequal in size. This need not be so and is only presented as such foreasier understanding thereof. A set of leads 80 is provided across thefirst segment. Providing power across these leads will cause thetransducer to oscillate at a first frequency. Across the first twosegments are another pair of leads 81. Typically, the pair of leads 80and the pair of leads 81 will have one common lead. Providing poweracross the leads 81 will cause the transducer to oscillate at a secondother frequency. Similarly, leads 82 and 83 are used for a third andfourth frequency, respectively.

[0081] Since a length of the excited transducer portion affectsfrequency, the result is a variable frequency oscillator. This isefficient and therefore desirable. The variable frequency oscillatorallows use of a fundamental frequency and its harmonics. Also, thetransducer segments can be excited in parallel. A single segment couldbe excited 8 times in parallel providing increased acoustic signalstrength.

[0082] The transducer segments may be non-identical as indicated above.This would allow for a binary type arrangement having three segments 1,2, 4 arranged in the following order (2)(1)(4) providing any of 1, 2, 3,4, 5, and 7 segment oscillators. Of course, other geometric methods ofvarying transducer frequency are also useful with the present invention.

[0083] Of course, numerous other embodiments may be envisaged, withoutdeparting from the spirit and scope of the invention. For example, thefirst and second transducers may be replaced with a plurality oftransducers. In this case, the plurality of transducers are supported ina manner that allows the applied force to be equally experienced as inthe general embodiment.

What is claimed is:
 1. A prodder having force feedback for detectingdetonatable devices or land mines, comprising: a reference object; a rodhaving an end for placing in contact with a first object to be detected;a first transducer coupled to the rod for providing an acoustic wave tothe first object and for receiving acoustic waves reflected from aboutthe first object; a second rod having an end for placing in contact withthe reference object; a second transducer coupled to the second rod forproviding an acoustic wave to the reference object and for receivingacoustic waves reflected from about the second object; and, anelectronics module for analysing data related to the waves reflectedfrom the reference object to determine a value related to the forceapplied to the first rod, for analysing data related to the wavesreflected from the first object in dependence upon the determined valueto determine acoustic characteristics of the first object material so asto categorize the first object's material in a force independentfashion.
 2. A prodder as defined in claim 1, wherein the firsttransducer, the second transducer, and the second object are arrangedco-linearly.
 3. A prodder as defined in claim 2, wherein the firsttransducer and the second transducer are piezoelectric crystalsspatially separated by an acoustically insulating material.
 4. A landmine detector having force feedback comprising: a second object withknown characteristics; a rod having a sensing end for contacting objectsburied beneath the ground; a second rod having a sensing end contactingthe second object with known characteristics; a transducer coupled to anon-sensing end of the rod for imparting mechanical energy into the rodtowards the sensing end in contact with a buried object, and fordetecting a signal corresponding to reflected waves from the buriedobject, the signal containing information related to a force exerted onthe rod and to material characteristics of the buried object; a secondtransducer coupled to an end of a second rod for imparting mechanicalenergy into the second rod towards another end in contact with thesecond object having known characteristics, and for detecting a signalcorresponding to reflected waves from the second object, the signalcontaining information related to the force exerted on the rod and tomaterial characteristics of the known object; and, means for processingthe signals, for determining at least one of the force exerted on therod, and force-independent material characteristics of the buried objectfor comparing with stored material characteristics of known objects soas to categorize the object's material.
 5. A land mine detector asdefined in claim 4 wherein the means for processing the signalscomprises means for determining both the force exerted on the rod andforce-independent material characteristics of the buried object forcomparing with stored material characteristics of known objects so as tocategorize the object's material.
 6. A land mine detector as defined inclaim 4, wherein the rod is non-magnetic, and wherein the land minedetector includes an indicator for indicating to a user when excessiveforce is being applied to the rod.
 7. A land mine detector as defined inclaim 6, wherein the indicator is a visual indicator.
 8. A land minedetector as defined in claim 6, wherein the indicator is an audioindicator.
 9. A land mine detector as defined in claim 4, wherein theforce is pressure.
 10. A land mine detector as defined in claim 4,wherein the transducers are piezoelectric crystals.
 11. A land minedetector as defined in claim 9, wherein the object having knowncharacteristics and the second transducer are disposed within theprodder in a manner such that, during use, each experiences a forceequivalent to the force with which the first rod is applied to theunknown object.
 12. A force feedback device as defined in claim 11,wherein the unknown object, the rod, the second transducer, and theknown object are disposed linearly.
 13. A force feedback device asdefined in claim 4, wherein the electronics module consists of a singleprocessor.
 14. A prodder having force feedback for detecting detonatabledevices or land mines, comprising: a rod having an end for placing incontact with a first object to be detected; a first transducer coupledto the rod for providing a first acoustic wave to the first object andfor receiving first acoustic waves reflected from about the first objectand for providing a second acoustic wave to the first object and forreceiving second acoustic waves reflected from about the first object;and, an electronics module for analysing data related to the first andsecond acoustic waves reflected from the first object to determineacoustic characteristics of the first object material so as tocategorize the first object's material in a force independent fashion.15. A prodder having force feedback as defined in claim 14 wherein theelectronics module comprises means for determining a value dependentupon applied force and object material characteristics from the firstaccoustic waves and a second value dependent upon applied force andobject material characteristics from the second accoustic waves, thevalues when corresponding to a same force and a same material indicativeof both the material characteristics and the applied force.
 16. Aprodder having force feedback as defined in claim 14 wherein thetransducer comprises an acoustic transducer having a plurality ofsegments and comprising means for exciting at least a portion less thanthe whole of the transducer corresponding to one or more segments forproviding a signal at a first frequency and another portion of thetransducer corresponding to a different one or more segments forproviding a signal at a second other frequency.
 17. A prodder havingforce feedback as defined in claim 16 wherein the acoustic transducer isa ceramic acoustic transducer.
 18. A prodder having force feedback fordetecting detonatable devices or land mines, comprising: a rod having anend for placing in contact with a first object to be detected; at leasta transducer coupled to the rod for providing acoustic waves to thefirst object via the rod and for receiving acoustic waves reflected fromabout the first object, the received reflected acoustic waves providingsufficient data for making a force independent measure of a materialcomposition of the first object; and, an electronics module foranalysing data related to the received acoustic waves reflected from thefirst object to determine acoustic characteristics of the first objectmaterial so as to categorize the first object's material in a forceindependent fashion.